Calcium is thought to be the principal physiological regulator of parathyroid hormone (PTH) secretion in vivo. In normal parathyroid tissue, there is an inverse sigmoidal relationship between PTH release and the extracellular calcium concentration, which provides a sensitive mechanism for regulating the secretion rate for PTH. In primary hyper-parathyroidism, on the other hand, many abnormal glands show a decreased sensitivity to the suppressive effects of extracellular calcium, which may contribute to the inappropriate hypersecretion of PTH in this disorder. Despite the importance of calcium in the regulation of normal and abnormal parathyroid tissue, little is known about the biochemical mechanisms by which changes in the extracellular calcium concentration modify parathyroid function. In the present proposal, we will evaluate the hypothesis that changes in cytosolic calcium concentration acting through the ubiquitous intracellular calcium receptor, calmodulin, play an important role in the regulation of PTH secretion in normal and abnormal parathyroid tissue. Specific aims are as follows: 1) Utilize the fluorescent, calcium-sensitive dye, QUIN2, as an intracellular probe to determine directly the cytosolic calcium concentration in dispersed parathyroid cells and its relationship to the control of PTH release by calcium, magnesium, and other ions. 2) Use QUIN2 to evaluate the role of changes in cytosolic calcium concentration in the regulation of PTH secretion by other secretagogues thought to act through cyclic AMP-dependent pathways. 3) Employ indirect immunofluorescence with an anticalmodulin antiserum to examine the intracellular localization of calmodulin in normal and abnormal parathroid tissue. 4) Evaluate the potential role of calcium-ATPases, putative, outwardly directed calcium pumps in parathyroid function by defining the subcellular localization, enzymatic characteristics, and regulation by calmodulin of these enzymes in normal and abnormal parathyroid tissue. 5) Examine the potential role of calmodulin in regulating the secretory process through direct actions on microfilaments and secretory vesicles. Evaluation of the roles of the cytosolic calcium concentration and calmodulin in PTH secretion may elucidate the biochemical mechanisms underlying normal PTH secretion and derangement in these normal mechanisms which lead to abnormal regulation of PTH release by calcium in pathologic parathyroid tissue.